Inhomogeneity of the radio frequency (RF) field causes variation of flip angle with spatial location. This flip angle variation may result in variable image intensity or failure of flip-angle-sensitive techniques such as fat saturation.
Measurement of flip angle can be used to correct image intensity variation caused by B1 field inhomogeneity. Measurement of flip angle is also useful to obtain correct results in T1 mapping with multiple-flip-angle techniques. Measurement of both flip angle and relative phase of the B1 field is also necessary for the design of RF waveforms for multiple simultaneous transmitter systems.
Mapping of the B1 field has been performed using double angle methods. In these methods, two spin-echo or gradient recalled echo (GRE) acquisitions are performed. In a spin-echo double angle method, a first acquisition M1 is performed with a α-2α sequence, and a second acquisition M2 is performed with a 2α-4α sequence. Dividing the magnitude of the first acquisition by the magnitude of the second acquisition gives a function of flip angle α as follows:
                                          M            1                                M            2                          =                  1                      8            ⁢                          cos              3                        ⁢            α                                              (        1        )            from which the flip angle α can be calculated asα=sec−1{square root over (8M1/M2)}  (2)
In GRE double angle methods, two GRE acquisitions are performed with flip angles α and 2α. Dividing the magnitudes of the two acquisitions gives a function of flip angle α as follows:
                                          M            1                                M            2                          =                  1                      2            ⁢            cos            ⁢                                                  ⁢            α                                              (        3        )            Equations (1) and (3) assume that the repetition time (TR)→∞. 
Other B1 mapping methods which derive estimates of B1 from signal intensities have been developed. One method utilizes two acquisitions with the same flip angle but alternating variable short sequence repetition time (TR). Another method uses three GRE acquisitions with flip angle distributed around 180° and fits the observed signal intensity to the expected 180° signal null. In another technique, flip angle is calculated from the ratio of a spin echo and a stimulated echo, similar to double angle methods but with both echoes obtained during the same sequence repetition.